The present invention relates generally to magnetic resonance imaging, and more particularly, to an improved timing sequence of controlling gradient signals relative to rf pulses to provide a system for magnetic resonance imaging of moving fluids.
NMR imaging as a medical diagnostic tool offers a number of important advantages over the various other means available for probing the human body. The most significant of these advantages result from the completely non-invasive nature of the technology, and the ability to obtain spatially encoded specimen data with a high degree of precision. Additionally, NMR has minimal, if any, hazards for either patients or operators of the apparatus; and perhaps most importantly, NMR image intensities are increasingly being found to be sensitive to various disease states. Clinical studies have shown that the relaxation time of malignant tissues are in general longer than those of the tissues of origin. This property is apparently not unique to cancerous tissue, but rather is indicative of the changes in molecular level structure of water associated with certain disease states. Other pathologies of NMR imaging include hydrocephalus; a carotid artery aneurysm; edema associated with kidney transplants; liver cirrhosis and several others.
In general, known NMR techniques for imaging of body tissue have tended to be of somewhat limited image quality, spatial resolution, and have required comparatively long imaging times to complete. In view of these technical and medical considerations, it is clearly of importance to improve NMR imaging technology by all available means. In particular, signal to noise ratios need to be increased, imaging times need to be shortened, spatial resolution needs to be enhanced, and imaging of the transverse and/or longitudinal relaxation times need to be accomplished. Many of these factors are not mutually exclusive. As a result, there has been a tendency to trade an improvement in one factor to the detriment of others. These trade-offs are not always salutary, further pointing to the need for basic improvements in NMR methods and apparatus.
Magnetic resonance imaging is presently being used to cover an increasing range of NMR techniques wherein static magnetic fields (to produce polarization of nuclei) are combined with field gradients (to spatially encode the sample volume of interest) and with rf fields (to spatially reorient polarized nuclei) to achieve a wide range of objectives, including imaging. In the recent past, the technical and patent literature have burgeoned reporting results of successive advances in the field. While the field has progressed steadily, certain intrinsic drawbacks have heretofore precluded the use of NMR high resolution imaging in medicine. Chief among these are comparatively slow relaxation times of human tissue, and body motion due both to inherent movements within the body as well as the difficulty of keeping the body stationary for long periods of time.
Biological tissue is known to have longitudinal (or spin-lattice) relaxation times T1, and transverse (or spin-spin) relaxation times T2, in the range of 0.04 to 3 seconds. Both of these time constants are exceedingly long as compared to the speed of the instrumentation presently available to process NMR signals. Also, high resolution imaging requires a large number of pixels, each of which may be the result of a complete NMR pulse projection, where each NMR projection is at least influenced by if not limited by these long time constants.
The signal obtained with steady-state pulse sequences in imaging depends on the magnetic field experienced by a spin in addition to intrinsic nuclear magnetic resonance parameters such as spin density, T1 and T2. This means that two spins with identical NMR parameters can have signal differences just because they experience different magnetic fields imposed by unavoidable magnetic field inhomogeneities. This resonant offset angle dependence of the signal can be reduced by minimizing the sequence repetition time, which has become possible with rapid advances in the data acquisition and gradient hardware capabilities.
In certain applications, it may be desirable to image moving fluids. Attempts using known methods results in voids in the images or incomprehensible images.
Current methodologies for steady-state gradient-echo pulse sequence design only insure that the zeroth moments (time integral of the gradient waveform over any TR period) of imaging gradients are nulled over each repetition period (TR). This results in a velocity dependence of the resonant offset angle (xcex2) of moving spins, which in turn results in poor image quality for the moving fluids.
It would therefore be desirable to provide a method for imaging that is robust enough to handle a reasonable range of magnetic field inhomogeneities that also reduce the velocity dependence of the imaging signal.
It is therefore one object of the invention to provide a simple steady-state imaging pulse sequence design method to ensure that the resonance offset angle (resonance offset angle xcex2=xcex3xcex94BTR; xcex2: phase angle accumulated by off-resonance spins during each repetition period) is independent of the velocity of spins in moving body fluids. This will result in a uniform steady-state signal response for a set of moving spins that have a distribution of velocities, which is typical of most physiological flow patterns.
In one aspect of the invention, an imaging apparatus particularly suitable for nuclear magnetic imaging has a transmitter coil generating a plurality of a first rf pulse and a second rf pulse having a repetition time therebetween. A first gradient coil is used to generate a first gradient waveform substantially centered within the repetition time. A second gradient is used to generate a second gradient waveform substantially centered within the repetition time. A receiver coil receives a resonance signal substantially centered within the repetition time. A controller is coupled to the receiver coil and forms an image in response to said resonance signal.
In a further aspect of the invention, a method of imaging comprises the steps of:
applying a first rf pulse and a second rf pulse having a repetition time;
generating a first gradient waveform substantially centered within said repetition time;
generating a second gradient waveform substantially centered within the repetition time;
receiving a resonance signal substantially centered within the repetition time; and
forming an image in response to said resonance signal.
One advantage of the invention is the inherent velocity compensation property which allows images of moving fluids. Another advantage of the invention is that the recovery of velocity compensated components of the signal acquired at a few resonant offset angles between zero and 2xcfx80 allows the use in imaging the cervical spine where the cerebrospinal fluid flows significant enough to cause signal voids in other known imaging systems. Yet another advantage of the invention is that because of high resolution capability, imaging of such structures as the inner ear including the cochlear cavity and cranial nerves in the cerebello-pontine angle may be achieved.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.